1. Technical Field
The present invention relates to an induction method for heating an electrically-responsive, temperature-sensitive element exposed to a fluid flow whereby a measurement of the removal of heat by the fluid flow alters the electrical characteristics of the element to produce a signal useful in determining the flow rate of the fluid and heat transfer to the fluid. The present invention further relates to a means for modulating the flow rate of the fluid in contact with the element to improve the precision of measurement.
2. Discussion
Mass flow sensors of the thermal type typically employ a tube which is electrically heated to reach a specific temperature above the temperature of the fluid flowing through the tube. A measurement of the electrical energy required to maintain this temperature difference while compensating for insulation losses, indicates mass flow rate of the fluid. The tube may incorporate an electrically insulated and thermally coupled wire coil on its outside surface through which an electric current flows. The wire may be of a temperature-sensitive type whereby its electrical resistance can serve also to indicate tube temperature or a separate sensor may be used for this function. It is likely that there will be at least some temperature difference present between the tube surface exposed to the fluid flow and the temperature sensor regardless of the form which the sensor may take. This is because of insulation and conductive losses of the sensor and the thermally insulating barrier between the tube and sensor, which are necessitated by the need to electrically insulate the sensor from the tube. This temperature difference is a source of flow measurement error. The temperature difference will be relatively large under the condition of high heat removal rates, as may be experienced with high velocity gases such as on steam or compressed air lines, or liquids such as water, and may therefore be a source of substantial flow measurement error. Furthermore, the overall thermal mass of the added heating wire and sensor increases the response time to changes in fluid flow rate and, in some applications, for example when used in a control system to regulate flow rate, may be a large factor in the overall system responsiveness.
The reduction in thermal mass is an advantage when flow modulation techniques are employed because the modulation frequency is limited by the response time of the heated tube. A higher modulation frequency enables the sensor to respond more quickly to changes in flow rate.
The present invention is further applicable to any instrumentation using a fluid-contacted surface with an objective being to heat the surface and measure the temperature change of that surface due to the effects of contact with the fluid, and where additional factors such as a thermally insulating coating or film on the fluid-contacted surface would otherwise affect the heat transfer between the surface and fluid.